How to Wire 50 Amp Service for an RVby John Cagney Nash
Although a 50 amp service for a recreational vehicle, or RV, is connected to four wires and uses a four-prong plug, it is still a three pole service with only hot, neutral and ground connections. It is different from a conventional 120 volt service in that there are two 120 volt hot feeds, or legs, each at 50 amps. A 50 amp service is not a 30 amp service and a 20 amp service joined together. Even the largest RVs, when plugged into 50 amp service, still almost universally use this double-hot-pole, or double-bus, installation to draw only 120 volts to two separate 120-volt circuits simultaneously, thereby accessing a possible 12,000 watts of power.
The outlet is called an NEMA 14-50R. Buy a 50 amp outlet which is pre-installed into product-specific, self-contained, weather-proof box. Buy ROMEX® #6 wire, meaning each hot wire and the neutral wire are six-gauge.
Disconnect the breaker panel which will serve the new 50 amp outlet by shutting off the main breaker. The supply to the breaker board will typically be split single phase, meaning two hot wires serve the breaker board along with a single neutral connection and a single ground connection.
Locate an unused double-pole 50 amp breaker, or install a double-pole 50 amp breaker in a vacant location. Connect the red wire to one terminal on the outlet side of the breaker, and connect the black wire to the other terminal on the outlet side of the breaker. Connect the white wire to neutral bus-bar. Connect the green or bare wire to the grounding block.
Wire the “U” shaped receiver, properly called the “half-round,” which is at twelve o' clock on a clock face, to ground, a green or bare wire. The terminal screw will be painted green.
Wire the receiver directly below the half-round, which is at six o' clock on a clock face, to neutral, a white wire. The terminal screw will be painted white.
Wire the two receivers to either side of the plug, which are at three and nine o' clock on a clock face, to the two hots, a red wire and a black wire. The two hot wires are phased at 180 degrees from each other, which is how twin 120-volt supplies are delivered to the RV rather than one 240-volt supply. The red and black wires are interchangeable, meaning that it does not matter which hot receiver either wire is connected to.
Switch the main breaker back on, and switch on the double-pole breaker serving the new 50 amp outlet.
Use your voltage meter, set on 240 volts as per the manufacturer’s instructions, to test the outlet before plugging the RV in for the first time. Locate one probe in one hot receiver and the other probe in the neutral receiver; the reading should be 120 volts. Locate one probe in the other hot receiver and keep the probe in the neutral receiver; the reading should still be 120 volts. Locate the two probes in the two hot receivers, one probe to each receiver, and the reading should be 240 volts.
- Treat electricity with respect. Mistakes can be lethal.
- Do not confuse the 14-50R with the 240-volt household outlet familiar from its use in powering clothes dryers and cookers. They are “common” outlets, meaning the two 120-volt supplies work together to deliver 240 volts to a 240-volt appliance. The 14-50R is an “uncommon” outlet, meaning it delivers two 120 volts currents. Your RV must be thought of as one very large 120-volt appliance. Even certified electricians who are unfamiliar with RVs can mistakenly assume an enormous RV is a 240-volt appliance, but plugging it into a 240-volt supply will destroy all appliances and accessories, and can cause fire. A wrongly-wired 50-amp service can introduce 240 volt electricity to your RV with the same results.
Items you will need
- NEMA 14-50R outlet
- ROMEX® six-gauge wire
- Basic electrical toolkit
- motorcoach,motorhome,rv image by Greg Pickens from Fotolia.com